What was found
APU FAULT LIGHT ON START UP APU DOES NOT START. GROUNDED - YES. CONFIRMED FAULT, APU WILL NOT START. REMOVED & REPLACED APU (SCU) START CONVERTER UNIT PER B737-800 AMM 49-41-61. ACCOMPLISHED APU BITE PROCEDURE PER AMM 49-61-00-700-801, OPS CK NML. ALSO, STARTED APU (WITH GPU) FIVE ATTEMPTS, OPS CK NML. STARTED APU (WITH GPU DISCONNECTED) FIVE ATTEMPTS, OPS CK NML. GOOD APU FREQ & VOLTS (400 HZ & 116V AC), BATTERY VOLTAGE 29VDC & 31VDC.